Quantity
Quantity is the number of followers, likes, views or other units you order, which must fall between the service's minimum and maximum and sets what you are charged.
The charge is the rate per 1000 multiplied by the quantity and divided by 1,000, taken from your balance when the order is placed. Statuses refer back to it: completed means the supplier reported the full quantity, and partial means it reported a shortfall, which is refunded at the same rate. A refill check later compares your count against the start count plus the quantity.
The unit is whatever the service defines, and it is not always obvious. On a watch hours service the quantity is hours, not views. On a custom comments service it is the number of comments, and on many panels it is set by the number of lines you paste in, so a stray blank or extra line changes what you pay. Some services sell a fixed package, and the quantity field is locked.
Drip feed changes the arithmetic. On many panels, with drip feed switched on, the quantity field is the amount per run, and the total delivered and charged is that quantity multiplied by the number of runs. A buyer who types the total into the quantity field and then sets ten runs orders ten times as much as intended.
What it does not mean
Quantity is not the number that will still be on the profile later. It is the number ordered, and it says nothing about drops after delivery. It also does not include any overdelivery the supplier adds, which is not charged but not promised either.